Microscopic mechanical and electrical devices of various sizes are used in a wide variety of applications. Continual shrinking of the size of integrated circuit (IC) elements demands a corresponding scaling of the processes and structures needed for the design, construction, and testing of scaled-down computer components. Related to the miniaturization of IC chips is the fabrication of microassemblies for use in MEMS (Microelectromechanical systems). MEMS are frequently fabricated on semiconductor substrates using processes similar to those used for IC fabrication.
Producing microscopic metallic structures can be difficult and expensive. One known technique for forming high aspect ratio metallic structures is referred to as “LIGA,” a German acronym for (X-ray) lithography (Lithographie), Electroplating (Galvanoformung), and Molding (Abformung). In a typical LIGA process, an X-ray sensitive photoresist material is deposited onto an electrically conductive substrate and exposed to highly collimated X-rays through a patterned mask. The areas that are exposed to the X-rays are chemically modified by the X-rays and can be dissolved in a developer, leaving patterns in the resist material corresponding to the area not exposed to the X-rays. The spaces in the pattern are filled by electrodeposition of a metal. The remaining resist is then removed, and the metallic pattern is used as a mold for injection molding to produce ceramic or polymer micro-parts. The LIGA process can also be used to make sacrificial plastic molds for the fabrication of metal micro-parts. The LIGA process requires the use of highly collimated X-rays, typically from a cyclotron, which makes the process expensive.
Another process for depositing a metallic conductive track on a substrate is ion beam-induced deposition (“IBID”), in which a precursor gas adsorbs onto a substrate surface and decomposes in the presence of the ion beam to deposit a metal on the substrate. Volatile products of the decomposition are removed by the system vacuum pump. Ion beam-induced deposition is used, for example, in the field of “circuit edit,” in which an integrated circuit, typically a prototype, is modified to add or remove electrical connections. The precursor gas can be a metal-organic compound, such as tungsten hexacarbonyl (W(CO)6). The energy for the deposition is thought to be transferred to the adsorbed precursor by lattice vibrations. Thus, not only will precursors at the beam impact point decompose to deposit material, but precursor molecules sufficiently close to the beam impact to be affected by the lattice vibration will also decompose. The primary ion beam also causes the emission of secondary particles, which can also cause deposition away from the impact point of the ion beam. Thus, even with a tightly focused beam, the minimum size of the deposited feature is still limited.
U.S. Pat. Pub. No. 20050227484 of Gu et al. for “System for modifying small structure,” which is assigned to the assignee of the present invention, teaches using ion beam-induced deposition to provide a conductive layer, and then electrodepositing another conductive layer on top of the ion beam-induced deposition (“IBID”) layer. The electrodeposited layer typically has better conductivity than the IBID layer. The electrodeposited layer, however, is at least as wide as the IBID layer. Thus, the combination of IBID and electro-deposition has at least the same feature size limitation as IBID.
Another difficulty caused by the miniaturization is the difficulty in testing circuits. Integrated circuits are often mounted into a package using a technique called “Controlled Collapse Chip Connection” or “C4.” The integrated circuit is mounted “upside down” on a substrate, with electrical connectors from the chip making contact with mating contacts on the package substrate. Such chips are therefore also referred to “flip chips.” To test such chips after they are mounted requires removing much of the silicon from the back of the chip to get close to the active circuit elements.
One method of determining the signal in a circuit element, such as a transistor, entails shining a laser upon the circuit element, and observing the effect of the current on the reflected light. Such a technique is described, for example, in “Novel Optical Probing and Micromachining Techniques for Silicon Debug of Flip Chip Packaged Microprocessors,” Paniccia et al., Microelectronics Engineering 46, pp. 27-34 (1999). As circuits become smaller, however, the laser is unable to focus to a sufficiently small area to determine the effects of a single transistor. Infrared laser-based tools are failing to scale due to wavelength limitations—the laser spot now encompasses multiple transistors in the region, making it difficult to determine the properties of a single transistor.
A proposed method of circuit edit for flip chips is described in “Contacting Diffusion with FIB for Backside Circuit Edit—Procedures and Material Analysis” by Kerst et al. STFA 2005, Santa Clara Calif., USA, Proceedings of the 31st International Symposium for Testing and Failure Analysis, pp 64-69 (2005), (“Kerst”). Kerst describes milling a trench from the backside of a wafer to contact either the diffusion region of a transistor or the contacts to the diffusion region using ion beam-induced deposition. The interface between the material deposited by ion beam-induced deposition and the doped semiconductor of the diffusion region produces a Schottky diode, rather than the preferred ohmic contact. Kerst describes a procedure for producing a silicide layer to provide an ohmic contact between a deposited FIB conductor and the doped silicon in the diffusion layer. The process requires heating the contact area, which can damage the integrated circuit.
Thus, there is a requirement for producing smaller metallic structures, both as free standing metal micro-part and as conductors on substrates.